Composite pallets, i.e. pallets which consist of different materials, have already been known in the state of the art for a long time. Thus a plurality of plastic pallets with metal inserts, such as are described for example in European patent no. 2 722 285, is offered. The metal inserts here ensure an additional reinforcement of the base and increase the resistance to mechanical loads on the base side.
In German patent application 35 04 314, a composite pallet made of wood or plastic with metal is described, with inner parts made of wood or plastic, which are formed as load-transmitting elements, and metal frames, which surround the support strips of a lower deck and an upper deck.
In some fields of industry, pallets are required which must be designed for the storage of containers over a very long period, in particular in terms of the storage of solid or liquid substances in barrels. Thus, for example, in the field of whisky production, selected whiskies are stored in wooden barrels over a period of 12 to 18 years. In many other branches, the long-term storage of barrels or containers in general also plays an important part. For this, wooden barrels are typically placed on pallets, several pallets with barrels are stacked one on top of another, wherein the under sides of each of the upper pallets lie on the barrels.
By a barrel is meant here, as a rule, a cylindric or cylindrical container produced from wood, metal or plastic, which can also be formed with a bulge. Barrels without a bulge are also called drums. Such a barrel usually serves to store liquids and as a rule is then filled via a bunghole, but it can also serve to store solid or powdered substances and is then, as a rule, provided with a lid. The shape of conventional barrels serves primarily to simplify transport by rolling, with the result that if the barrels are not rolled but transported by machines for example, other container shapes also fulfil the function of a barrel, for example cuboid containers, which in this sense can also be called barrels.
Wooden pallets, which have the necessary long-term stability if the wood is defect-free, are usually used for storing barrels.
However, the use of wooden pallets has disadvantages. For example, over time, wooden pallets acquire black marks if wooden barrels with spirits are stored on them. These marks result from the gas release of alcohol from wooden barrels. In addition, it is possible that the wood will warp over the years because as a rule, long-seasoned, expensive wood is not used in pallet production. This can lead to instabilities in the storage in particular of pallets with barrels stacked one on top of another. Finally, the use of plastic pallets allows a greater flexibility, which can be provided for example via different inserts for the shapes to be used for production. In principle, the durability of plastic pallets, unless they are subjected to continuous loading at isolated points, is also greater than that of wooden pallets.
However, when considering long-term stability, it is to be borne in mind that the barrels—in particular those with a stepped rim at the top and bottom—subject the pallet on which they are stored to a continuous, if spatially narrowly limited, loading. If plastic is used as a material for the pallets, such a constant long-term loading always at the same place results in a creep, in a retardation, which can greatly impair the stability of the whole pallet and the stableness of the barrels stored thereon, for which reason plastic, despite its advantageous properties, has not yet been used for pallets which are provided for the long-term storage of barrels—i.e., over a period of at least 12 years.